I agree with you that De Vito probably was against the Rizutto's even though there has been some doubt cast on this. However, I've never seen anything suggesting he was aligned with Montagna. I feel like I remember the things I've read suggesting he was working more with Desjardins/Mirarchi.
I'd be interested to see your sources on his alignment with Montagna. I'd also like to see the sources that believe he was behind the Renda disappearance and the Agostino hit... I need something to do for the next couple of days